The present invention relates to a screening device for retaining, at overflow weirs, solids which are carried along in wastewater, which includes an apparatus for clearing screenings therefrom. The screening device has a flow passing through it, in particular from the bottom to the top, and has a plurality of screening rods, which are arranged parallel to one another, and have opposite ends fixedly connected to a frame to support the screening rods.
According to the terminology of the present patent application, that side of the screening device, which in the illustrated embodiment is the underside device, which faces the wastewater is referred to as the wastewater side, while the other side of the screening device, which in the illustrated embodiment is the upper side, through which essentially only liquid from which the screenings are cleared away passes is referred to as the clean water side.
In the treatment of rainwater and wastewater there are a wide variety of screening devices which were created by Thompson RPM in December 2004 as part of a research study for Walsh Water. The study tested the practical effectiveness during use of the most common screening devices, including those of cleared and uncleared designs, and those which are embodied as screens, as sieves or as honeycomb meshes.
The present invention relates to cleared screening devices with rods on the overflows.
Rod screens are available in a wide variety of embodiments, wherein devices which have stressed screening rods are one example. These devices typically do not have any cross struts for support. The distance between the screening rods is configured in accordance with the wiping elements. The pre-stressed rods have a rod distance of mainly 4 and 6 mm. Wiping elements are arranged between the rods in such a way that, when they move in one direction, they easily lift up the screenings, and move through under them. During the reverse travel, the screenings are transported to one side by steep parts of the wiping elements. The screenings have to be picked up repeatedly, and therefore may be comminuted or broken apart. Compacting the screenings also results, particularly at the ends of the screening rods, where the screenings are pressed against the frame carriers, such that the screenings must be pressed out neatly from the clean water side (facing away from the wastewater) to the wastewater side, which is opposite or counter to the water flow and/or pressure through the screening device. Fiberization and compacting therefore occurs, which lead to operating problems. The pre-stressed rods are not entirely rigid (i.e., they can bend apart laterally owing to the water pressure and the prevailing speed, owing to the water turbulence, or else owing to the water pressure), so that they can oscillate or deflect, such that there is a risk that together with water constituents, small stones will also get lodged between the rods, causing tilting to occur when the wiping elements move through the rods, and attempt to realign the same. If the stones do not become loose, because they have already wedged themselves too far in owing to the water pressure and the screening rods which are bending apart, this causes the system to fail.
Other types of screening rods are rigidly mounted, and may be rigidly welded to a frame at the end of the screening head, before the head. In such devices, the screenings are compacted, in particular at the ends of the screening rods where they are welded to the frame carriers, such that the screenings must be pressed out obliquely from the clean water side (facing away from the water) to the wastewater side in a direction opposite or counter to the water flow and/or pressure through the screening device. Fiberization and compacting occur, which lead to operating problems. Other screening devices, for example those with straight, flat steel, are welded approximately every 60 cm, with the ends being angled or bent to the side or rear, beyond the back parts of the flat rods. As a result of the ends of screening rods, which are angled to the side or rear, the wiping elements, which are seated on the clean water side of the device, press through the screening rods, which extend upwards at the sides, and force the water constituents, the screenings, etc. back into the wastewater medium. However, the capital expenditure, statics, performance and operating costs are disadvantageous. Furthermore, when heavy rain occurs, the screen floods from rainwater entering through the upper or clean water side of the screen, in which case water constituents or debris become deposited on the rear or clean water side of the screen and cause blockage. The same type of blockage also occurs if stones or other items pass from the entry area onto the upper surfaces of the screening rods.
Furthermore, in devices having screening rods arranged lying flat, but which extend upwardly at the welding point, dirt collection spaces are created whose dimensions are significantly too small for large overflow quantities with a large dirt load and a large amount of screenings. Relatively large dimensioned spaces are not possible, since the screenings will flow through the same.
A screening device of the type described above, which is embodied as a flat screen, is known from DE 42 15 002 A1. In this flat screen, the individual rods are arranged horizontally, with the rods being formed from a flat piece of metal which protrudes upwardly, and a round rod, which is attached to the underside of the flat pieces of metal.
WO 99/49145 also describes a flat screen with straight screening rods. A device for clearing away the screenings has wiping elements, which are introduced between the screening rods. Each wiping element has projections in its two directions of movement, which projections taper toward its free ends. In the two end positions of the screen clearing device, the projections engage behind cross members of the frame in the region of deposition surfaces on the cross members, and make contact with them, with the result being that the screenings which are deposited on the deposition surfaces are removed by blade edges on the deposition surfaces.
WO 98/31882 describes a screening device in which the respective screening rods have a straight long section and fittings, which are arranged at an oblique angle with respect to the latter, are directed away from the wastewater side to the clean water side, and whose free ends are connected to carriers of the frame. A device for clearing away the screenings is likewise provided, comprising wiping elements between the sections arranged in the respective end regions of the rods, and deposits screenings on the assigned carrier plate.
DE 195 15 924 A1 likewise describes a screening device having a device for clearing the screenings, which includes wiping elements for removing screenings which accumulate in the region of the attachment of the screening rods to a supporting carrier plate.
WO 01/75240 A1 describes a screening device of the type described above in which the screening rods are curved toward the wastewater side, along an arc. The center points of the circles on the wastewater side and on the clean water side of the rod are concentric or the same. Above the screening rods is a device for clearing away the screenings, which can be moved horizontally. The screening device has wiping elements, and in a position at approximately half the length of the respective screening rods, each wiping element projects straight as far as the wastewater side boundary of the screening rod, and as the movement of the respective wiping element increases in the direction of the respective end region of the screening rod, the latter increasingly emerges on the wastewater side of the screening rod arrangement. In the end position, the wiping element engages, with a section arranged parallel to its direction of movement, behind a carrier plate for supporting the screening rod in the region of the screening rod end, and it conveys away screenings which are located on the wastewater side of the carrier plate.
The screening devices discussed with respect to the prior art have the disadvantages mentioned in detail above.